For the following crosses, determine as accurately as possible the genotypes of each parent, the parent in whom nondisjunction occurs, and whether nondisjunction takes place in the first or second meiotic division. Both color blindness and hemophilia, a blood-clotting disorder, are X-linked recessive traits. In each case, assume the parents have normal karyotypes.
A color-blind man and a woman who is wild type have a daughter with Turner syndrome (XO) who has normal color vision and blood clotting.

Verified step by step guidance

1. Step 1: Understand the genetic basis of the traits and the chromosomal abnormality. Color blindness and hemophilia are X-linked recessive traits, meaning they are carried on the X chromosome. Turner syndrome (XO) results from a missing X chromosome, leading to a 45,X karyotype.
2. Step 2: Analyze the genotypes of the parents. The man is color-blind, so his genotype is X^cY, where X^c represents the X chromosome carrying the color-blind allele. The woman is wild type, meaning she has two normal X chromosomes (X⁺X⁺).
3. Step 3: Determine the possible gametes produced by each parent. The man can produce gametes with either X^c or Y. The woman can produce gametes with X⁺.
4. Step 4: Consider the nondisjunction event. Since the daughter has Turner syndrome (XO) and normal color vision, she must have inherited one X chromosome from her mother (X⁺) and no sex chromosome from her father. This indicates nondisjunction occurred in the father during meiosis.
5. Step 5: Identify the stage of nondisjunction. If nondisjunction occurred in the first meiotic division, the father would produce gametes with both X^c and Y or no sex chromosome at all. If it occurred in the second meiotic division, the father would produce gametes with either X^c, Y, or no sex chromosome. Since the daughter inherited no sex chromosome from the father, nondisjunction likely occurred in the second meiotic division.

Key Concepts

X-linked Inheritance

X-linked inheritance refers to the pattern of inheritance for genes located on the X chromosome. In this case, color blindness and hemophilia are X-linked recessive traits, meaning that males (XY) are more likely to express these traits since they have only one X chromosome. Females (XX) can be carriers if they have one affected X chromosome but typically do not express the trait unless both X chromosomes are affected.

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to gametes with an abnormal number of chromosomes. This can occur during either the first or second meiotic division. In the context of Turner syndrome (XO), nondisjunction likely occurred during the formation of the egg or sperm, resulting in a missing X chromosome in the daughter.

Turner Syndrome

Turner syndrome is a chromosomal disorder characterized by the presence of a single X chromosome (XO) in females, leading to various developmental and physical features. Individuals with Turner syndrome typically have normal intelligence but may experience short stature, infertility, and other health issues. In this scenario, the daughter with Turner syndrome has normal color vision and blood clotting, indicating that she inherited a normal X chromosome from her mother.